The present invention relates to a measurement apparatus for measuring a physical quantity such as fluid flow, pressure, temperature, O2 concentration, etc., and especially to an air flow-measurement apparatus for measuring the flow rate of air taken into an internal combustion engine.
A thermal type flowmeter has been a mainstream of an air flow sensor, which is located in an intake-air passage in an internal combustion engine of a vehicle such as a car, because this type air flow sensor can directly detect a mass flow rate of air.
In the technique of the above thermal type air flow sensor, an auxiliary air passage is formed in an intake-air passage, and a heating resistor element and a temperature sensitive resistor element are situated in the auxiliary air passage as disclosed in Japanese Patent Application Hei 8-338745. Further, by providing radiation fins at the side face of the auxiliary air passage, the temperature of the air flow sensor is prevented from increasing du to the heat generated in an engine.
Moreover, an intake-air temperature sensor, which is located in an intake-air passage, for detecting the temperature of the intake-air, is disclosed in Japanese Patent Application Laid-Open Hei 6-160204. In the embodiments of this application, the temperature sensitive resistor is connected to a metal terminal, which are molded in one body by using a resin mold.
In addition, a sensor in which a part of its support member is a metal plate, is disclosed in Japanese Patent Application Laid-Open Hei 11-14423.
The conventional techniques have the following shortcomings. Usually, resin is used for members composing an auxiliary air passage, a housing, etc., in which a temperature sensitive resistor located in an intake-air passage, a heating resistor, an intake-air temperature sensor, etc., are situated. Since the thermal conductivity of resin is smaller than that of metal and ceramics, resin is proper in the light of preventing the heat generated in an engine from being conducted to the temperature sensitive resistor, the heating resistor, the intake-air temperature sensor, etc., via the outside wall of the intake-air passage. However, although the resin can prevent the temperature of the above sensors from increasing due to the heat conduction, it is not possible to completely prevent the temperature of the above sensors from increasing because the sensors receive the radiation heat from the internal wall of the intake-air passage. In the conventional techniques, the effect of this radiation heat is not absolutely considered.
On the other hand, since the emissivity of metal is greatly smaller than other material, metal is proper to prevent the temperature of the sensors from increasing due to the radiation heat. However, as described above, since the thermal conductivity of metal is large, if the whole of portions such as the auxiliary air passage or housing is made of metal, the heat is conducted to the temperature sensitive resistor, the heating resistor, the intake-air temperature sensor, etc., via the outside wall of the intake-air-passage, which in turn makes it impossible to prevent the temperature increase of the sensors.